In recent years, in order to reduce exposure of individuals to lead in their water supply systems, federal and state government agencies have issued regulations that provide standards for acceptable levels of lead in drinking water and the amount of lead that can leech from plumbing fittings. In order to meet these specifications, several low lead or lead-free alloys are now being employed in plumbing fixtures.
Plumbing fittings, such as valves, typically have valve bodies which are machined in two parts for ease of assembly. Valves, such as ball valves, typically have a body section into which the valve ball and valve stem are first inserted followed by an end cover which is threaded into the internally threaded valve body.
Typically, lead-free alloys are more difficult to machine than conventional lead-containing bronze alloys. Many low lead and lead-free alloys are abrasive, have higher tensile and yield strengths, require more costly tooling, result in shorter tool life upon machining and require increased energy consumption during manufacturing compared to lead-containing alloys. As a result, threads between the valve body and the end cover, when fabricated from low lead and lead-free alloys, are difficult and costly to machine in view of the material properties of these alloys. This, in turn, greatly increases the cost of manufacturing valves and various plumbing fittings in a traditional manner.
Engagement of low lead and lead-free copper alloy threads in fittings also presents challenges with respect to achieving and maintaining a good seal at the joints made with these threads. In contrast, the lead in leaded alloys would smear along the faces of the threads upon machining, thus providing a lead film that would lubricate and level out irregularities between mating surfaces. As a result, leaded alloys could facilitate higher thread engagement torques and excellent sealing capabilities. With the reduction and/or loss of lead in the low lead and lead-free copper alloy fittings, these inherent benefits of lead are lost. Not surprisingly, low lead and lead-free alloys can result in fitting designs with poor thread connections due to the relatively high strength and low ductility of these alloys. Further, these thread engagement issues with low lead and lead-free alloys become even more pronounced in high temperature applications, such as steam, where there is a potential for thermal expansion to impact mating components and create leak paths.
In general, welded joints are generally viewed as improvement over threaded joints in plumbing fittings and valves. While welding processes are generally understood to be lower in cost than machining processes used to make threaded features in valves, fittings and the like, welding has not been successfully employed to date to join valves, fittings and the like fabricated from lead-free and low lead alloys. Among other considerations, the low lead and lead-free alloys in such fittings and valves possess material properties that have inhibited the development and optimization of welding processes for these fittings and valves.
Weld joints employed with components fabricated from copper alloys typically exhibit a heat affected zone (“HAZ”). With regard to in-service corrosion resistance, the HAZ, when in contact with a corrosive media within the fitting (e.g., potable water), can exacerbate any leaching of alloy constituents from the fitting (e.g., small amounts of lead, other metals, and other constituents) into the corrosive media. Further, the HAZ itself can result in a degradation of the mechanical properties of the fitting, particularly portions of the fitting in proximity to the HAZ. Further, the HAZ of the weld joint can enhance the local corrosion rates of any portion of the HAZ in proximity to or contact with the corrosive media of the plumbing fitting.
Accordingly, there is a need for low lead and lead-free plumbing fitting designs for use with potable and non-potable aqueous media (and methods of making these fittings) with components that can be joined with welds that resist corrosion and exhibit high mechanical integrity. There is also a need for fitting designs, and welding methods for making these fittings, that optimize the size and location of the HAZ in view of the material properties of the components of the fitting and in-service corrosion resistance.